1. Field of Invention
The invention relates to a method for operating offshore wind turbine plants based on the frequency of their towers. More particularly, it relates to a method for operating offshore wind turbine plants wherein a critical natural frequency of the power plant if constantly determined, changes to a fixing strength are recognized and a forbidden resonance range threshold is correspondingly displaced.
2. Description of the Prior Art
Wind power plants, which at present are largely installed on land, are founded on foundation soil of varying strength. If the soil has an adequate bearing capacity flat foundations of concrete are adequate for static and dynamic requirements. If the surface area has inadequate bearing capacities, piles are introduced into the underlying load bearing layers in order to introduce into the soil the wind power plant loads. A deign criterion for the structural dimensioning of the tower ad foundation parts are the lowest tower natural bending frequencies.
The exciting frequencies of the rotor must in operation of the plant always have a certain spacing from the aforementioned tower natural frequencies, because otherwise there are dynamic superelevation of the structural loading leading to premature component fatigue failure. The exciting frequencies are the rotor speed and the blade multiple thereof. These dynamic superelevations as a result of resonances must be avoided, in order to achieve the intended mathematical service life of the load-transferring components of a wind power plant. Thus, through the structural dimensioning of the tower and the foundation of land-supported plants, the initial tower natural frequency is conventionally interpreted in such a way that under all operating conditions the exciting frequency must be adequately spaced from the tower natural frequencies.
In the mathematical interpretation of the necessary natural frequencies, account must be taken of the characteristics of the surrounding soil. These soil characteristics influence the rigidity of the foundation fixing and therefore the natural frequencies. In the case of land-supported supported plants in a first approximation there is no change over a period of time in the fixing rigidities of the foundation. Thus, the natural frequencies of the plant also remain roughly constant over the service life.
EP 244 341 A1 refers to the occurrence of resonances, which arise on reaching a specific rotor speed. It is proposed that this rage be avoided by rapidly passing through it.
In offshore plants anchored by one or more piles on the ocean bed, as a result of flow changes around the foundation structure, tidal currents or strong wave movements, the piles are flushed out to a greater or lesser extent.
This phenomenon, known as erosion, has the consequence of the fixing rigidity of the pile changing and, consequently, so does the tower natural frequency. In addition, the dynamics of the plant also lead to a change in the ocean bed surrounding the pile and, therefore, to a change in the natural frequency.
Moreover, in an offshore wind park, the soil conditions differ at each plant location. As the foundation parts are to have the same construction for cost reasons, the fixing rigidity and, therefore, the natural frequency differ for each plant. These changes and differences cannot be calculated in advance, are of an arbitrary nature, differ for each plant and are permanently variable over a period of time. As a result, with varying natural frequencies, the plants can be subject to superelevated operating strength loads ad premature failure occurs.